1. Field of the Invention
The present invention relates to an electronic odo/trip meter, and particularly to an electronic odo/trip meter having a non-volatile memory. Specifically, the present invention relates to an electronic odo/trip meter wherein travel distance data is stored into the non-volatile memory only when a power source is cut off or becomes less than a predetermined voltage.
2. Description of the Prior Art
Recently, various electronic odo/trip meters applicable in automobiles have been proposed and developed. One such electronic odo/trip meter for automobiles is disclosed in the Jidosha Gijutsu Vol. 38, Page 185-186, published on 1984. This type of electronic odo/trip meter is shown in FIG. 1.
As shown in FIG. 1, the odo/trip meter 100 comprises a micro computer 1, a read write memory (RWM) 2, and an electrically erasable programmable read only memory (EEPROM) 3. The micro computer 1 processes a speed pulse signal indicative of vehicular speed and determines the sum of travel distance which will be hereinafter referred to as travel distance data. The micro computer 1 rewrites the travel distance data stored in the RWM 2 every 0.1 km and rewrites the data stored in the EEPROM 3 every 0.8 km. The data of the RWM 2 and the EEPROM 3 are displayed through a display driving circuit 4 in a display panel 5, respectively. A read write memory 7 is applied to store trip data indicative of a short travel distance. A switch 6 is used for switching the data to be displayed on the display panel 5, namely the overall travel distance data and the short travel distance data. A switch 8 is used for resetting the travel distance data stored in the RWM 7.
Specifically, the EEPROM 3 is provided so as to prevent elimination of the travel distance data if the power source is cut off due, for example, to power switch OFF, electric discharge of battery, breakage of battery, etc. As set forth above, since the EEPROM 3 is updated every 0.8 km, the travel distance data stored in the EEPROM varies within a range of an error 0.8 km relative to the true distance. When the travel distance data is stored in the EEPROM 3, the travel distance data is simultaneously stored in three different addresses of the EEPROM 3. Therefore, when the travel distance data is derived from the EEPROM, the three travel distance data are derived from the three different addresses and then these data are processed in a majority operation of the micro computer 1, thereby producing a highly reliable data. As the travel distance data is rewritten frequently, preferably a plurality of memory addresses of the EEPROM should be used in order to avoid frequent use of a particular address so as to prevent degradation of the particular memory cells.
FIG. 2 is a prior art electronic odo/trip meter having an IC 10 which includes a control circuit 11 and a counter 12 and is connected to an EEPROM 13. The EEPROM 13 is external to the IC 10. The vehicular speed pulse signal is inputted into the IC 10 and is processed by the control circuit 11 with the result that the travel distance data is determined. As a result, the counter 12 counts up predetermined travel distance increments, for example 0.1 km. The enumerated value of the counter 12 is displayed through the display driving circuit 4 in the display panel 5. On the other hand, the enumerated value of the counter 12 is written into the EEPROM 13 at predetermined increments.
Essentially, since the EEPROM 3 or 13 is used for storing the travel distance data even if the power source is cut off, it would be highly desirable for the data stored in the RWM 2 or the counter 12 to be transmitted into the EEPROM 3 or 13 only when the power source is cut off. However, the prior art odo/trip meter employs an off the shelf commercial EEPROM, which demands relatively great deal of current. Therefore, when the power source is cut off, it is difficult to update the travel distance data in the EEPROM by means of a back-up power source because of the relatively the low capacity of the back-up power source. Therefore, as set forth above, the travel distance data of the EEPROM is rewritten at predetermined intervals. If the travel distance data is stored only in particular addresses of the EEPROM when the data is rewritten every predetermined distances, the particular memory cells most frequently used tend to deteriorate. Many electronic elements are required to distribute the work load among the addresses in the EEPROM.
The odo/trip meter can usually display a six figure decimal so as to give maximum displayable travel distance 99,999.9 km. 9 in the decimal system corresponds to 1001 in the binary system. Therefore, since six figures in decimal numerals corresponds to 6.times.4=24 figures in the binary system, a 24 bit memory is required in the RWM 2 and the counter 12. However, since commercial 8 bit EEPROMs are conventionally used as the non-volatile memory, the travel distance data are divided into several bytes and distributed among a plurality of EEPROMs.
In the above system, there is the likelihood that the power source may become cut off or lowered when the travel distance data are being stored in the memories of the EEPROM 3 and 13. Therefore, fail-safe circuit is required so as to prevent failure of the data transfer.
Thus, in the existing odo/trip meters, many extra electronic parts are required, thereby increasing the cost of the product and electric power consumption and limiting the compactness of the arrangement. Thus, the main purpose of the invention is to prevent failure of the data transfer and to provide a compact odo/trip meter.